Depends what you want of course. I did a quick Google, which might give you some ideas. Just outside the County, there's purpose built facility at Rowney Warren, near Shefford in Bedfordshire. And for easier riding there are old railways from Hatfield to St Albans, Hertford towards Welwyn, Hemel Hempstead to Harpenden, and south of Luton past Harpenden to Welwyn.